By reading servo information recorded within data tracks on a disk surface, a disk drive head positioner servo system is able to estimate data transducer head position. The recorded servo information typically includes track (i.e. cylinder and head) addresses and servo bursts. Each circumferential data track on a disk surface has a unique track address, which is recorded in servo sectors embedded in the track, and servo burst patterns frequently repeat every two or more tracks. When a disk drive is seeking to a radial track location, the track addresses are used as coarse positioning information to approximately estimate the position of the read head and the servo bursts are used as fine positioning information to position the head precisely on the desired radial location.
At seek time while reading track addresses, the head may be positioned between two adjacent tracks. In this situation, the head may receive a superposition of signals from both tracks. One solution to this ambiguity is to encode the track addresses into Gray-coded addresses so that the encoded addresses of any two adjacent tracks differ from each other by only in a single bit position. With this solution, when the head is reading two tracks, the ambiguity after decoding the address is one track, and an error of one track is acceptable at seek time.
In accordance with one known technique, each data track is divided into plural sectors. Each sector includes a header section, followed by a data section. The header section may typically include a DC erase field, a preamble field, a header synchronization character, a track address field (coarse servo information) and a servo burst field (fine servo information). The data section may typically include another preamble field, a data synchronization character, a block of user data, and error correction bytes. In this example, the header section is recorded at the same data rate as the data section, and synchronous peak detection through a single read channel structure in the disk drive is employed to read the information in both the header section and the data section. An example of this approach is found in commonly assigned U.S. Pat. No. 5,036,408 to Leis et at., entitled: "High Efficiency Disk Format and Synchronization System", the disclosure thereof being incorporated herein by reference.
Another known technique is to employ radial zones or bands of concentric data tracks, each zone having a data transfer rate associated with disk radius of the zone. In this example, data areas are separated by a series of radially extending embedded servo sectors which are factory recorded with servo information at a single data transfer rate. A servo data recovery circuit asynchronously (i.e. without phase lock to incoming servo data) recovers a servo address mark, a track number and fine position information from information read by the data transducer while passing over each sector. The servo recovery circuit is separate from the read channel electronics employed for peak detection of user data information. This example is described in commonly assigned U.S. Pat. No. 5,420,730 to Moon, et at., entitled: "Servo Data Recovery Circuit for Disk Drive Having Digital Embedded Sector Servo", the disclosure thereof being incorporated herein by reference.
One factor which has limited data storage densities in magnetic recording employing peak detection techniques has been intersymbol interference, arising when flux transitions are increasingly close to each other. One technique for increasing flux densities in magnetic recording while still accurately reading recorded data is to employ synchronous sampling data detection. This technique, frequently referred to as "partial response, maximum likelihood" (PRML) signalling, has provided some improved data storage densities, at the expense of increased circuit complexity, including a fast analog to digital conversion process, and channel equalization, either on the analog side or on the digital side of the signal stream, or both. An example of a disk drive employing PRML is given in commonly assigned U.S. Pat. No. 5,345,342, to Abbott et at., entitled: "Disk Drive Using PRML Synchronous Sampling Data Detection and Asynchronous Detection of Sector Servo", the disclosure thereof being incorporated herein by reference. The approach described in this patent enabled special circuitry within the synchronous sampling data detection channel to asynchronously detect track number values in embedded servo sectors recorded at a constant servo data rate whereas the user data rate differed by radial data zone across the recording disk. The servo bursts were read and processed using conventional peak detection, and sample and hold techniques.
An improvement over the asynchronous servo sampling technique taught by the Abbott et al. patent referred to above is found in a later, commonly assigned U.S. Pat. No. 5,384,671 to fisher, entitled: "PRML Sampled Data Channel Synchronous Servo Detector", the disclosure thereof being incorporated herein by reference. In this approach a timing loop of the synchronous sampling data detection system is phase locked to servo information, the servo information including track address and fine position information is synchronously sampled and decoded. In this approach the servo preamble field is recorded as a conventional 1/4T sine wave pattern, which corresponds to a 2T pattern in a peak detection channel (T representing a unit sample cell or interval).
Viterbi detectors are known for use within synchronous sampled data detection channels. One example of a Viterbi detector is provided in commonly assigned U.S. Pat. No. 5,341,387 to Nguyen, entitled: "Viterbi Detector Having Adjustable Detection Thresholds for PRML Class IV Sampling Data Detection", the disclosure thereof being incorporated herein by reference.
While these prior approaches have worked well in their respective times, increasing data storage capacities and data transfer rates per unit size disk have led directly to a hitherto unsolved need for an improved disk drive head servo format, architecture and approach.